Image sensors are used in cameras and other imaging devices to capture images. In a typical imaging device, light enters through an opening (aperture) at one end of the imaging device and is directed to an image sensor by an optical element such as a lens. In most imaging devices, one or more layers of optical elements are placed between the aperture and the image sensor to focus light onto the image sensor. The image sensor consists of pixels that generate signals upon receiving light via the optical element. Commonly used image sensors include CCD (charge-coupled device) image sensors and CMOS (complementary metal-oxide-semiconductor) sensors.
Filters are often employed in the image sensor to selectively transmit lights of certain wavelengths onto pixels. A Bayer filter mosaic is often formed on the image sensor. The Bayer filter is a color filter array that arranges one of the RGB color filters on each of the color pixels. The Bayer filter pattern includes 50% green filters, 25% red filters and 25% blue filters. Since each pixel generates a signal representing strength of a color component in the light and not the full range of colors, demosaicing is performed to interpolate a set of red, green and blue values for each image pixel.
The image sensors are subject to various performance constraints. The performance constraints for the image sensors include, among others, dynamic range, signal to noise (SNR) ratio and low light sensitivity. The dynamic range is defined as the ratio of the maximum possible signal that can be captured by a pixel to the total noise signal. Typically, the well capacity of an image sensor limits the maximum possible signal that can be captured by the image sensor. The maximum possible signal in turn is dependent on the strength of the incident illumination and the duration of exposure (e.g., integration time, and shutter width). The dynamic range can be expressed as a dimensionless quantity in decibels (dB) as:
                    DR        =                              full            ⁢                                                  ⁢            well            ⁢                                                  ⁢            capacity                                R            ⁢                                                  ⁢            M            ⁢                                                  ⁢            S            ⁢                                                  ⁢            noise                                              equation        ⁢                                  ⁢                  (          1          )                    Typically, the noise level in the captured image influences the floor of the dynamic range. Thus, for an 8 bit image, the best case would be 48 dB assuming the RMS noise level is 1 bit. In reality, however, the RMS noise levels are higher than 1 bit, and this further reduces the dynamic range.
The signal to noise ratio (SNR) of a captured image is, to a great extent, a measure of image quality. In general, as more light is captured by the pixel, the higher the SNR. The SNR of a captured image is usually related to the light gathering capability of the pixel.
Generally, Bayer filter sensors have low light sensitivity. At low light levels, each pixel's light gathering capability is constrained by the low signal levels incident upon each pixel. In addition, the color filters over the pixel further constrain the signal reaching the pixel. IR (Infrared) filters also reduce the photo-response from near-IR signals, which can carry valuable information.
These performance constraints of image sensors are greatly magnified in cameras designed for mobile systems due to the nature of design constraints. Pixels for mobile cameras are typically much smaller than the pixels of digital still cameras (DSC). Due to limits in light gathering ability, reduced SNR, limits in the dynamic range, and reduced sensitivity to low light scenes, the cameras in mobile cameras show poor performance.